1. Field of the Invention
The present invention relates to a double-pipe pipe coupling and, more particularly, an improvement of the pipe coupling intended to completely shut out outside air.
2. Description of the Related Art
The pipe coupling has been used for the heat medium circulating piping of the air conditioner, for example, but in the case of the pipe coupling of this kind, outside air must not enter into the pipe coupling in order to enhance the rate of heat exchange achieved by coolant. The pipe coupling has therefore the so-called airtight construction to prevent outside air from coming into it by some means.
However, the conventional pipe coupling had the airtight construction but it was not of double-pipe construction. It was therefore needed that the pipe coupling of one kind was used for the piping through which coolant was fed and that the pipe coupling of another kind for the piping through which coolant was returned. This needed wide piping space, caused the cost to become high and made it impossible to provide the piping at low cost.